1. Field of the Invention
The present invention relates to a fixing device which is mounted in an image forming apparatus such as a copying machine or printer and fixes a developer image on a paper sheet.
2. Description of the Related Art
Conventionally, an image forming apparatus such as an electrophotographic copying machine utilizing a digital technique comprises a fixing device which fixes a developer image onto a paper sheet by heating in a press state.
In recent years, a short warming-up time becomes a technical issue as an energy-saving technique. The measure is to decrease the diameter of the heating roller.
However, a small-diameter heating roller decreases the heat capacity, and it becomes difficult to keep the temperature distribution uniform on the heating roller. For example, an induction heating fixing device may generate a nonuniform temperature distribution on the heating roller unless power per unit area to the heating roller heated by a coil is set to a desired value. The fixing device used in an image forming apparatus may use paper sheets of various sizes, and the coil must be so designed as to set the temperature distribution to a desired one on the heating roller regardless of a paper sheet of any size. Otherwise, the fixing temperature for various paper sheets cannot be kept uniform.